The OmegaWhite Survey for Short-period Variable Stars. V. Discovery of an Ultracompact Hot Subdwarf Binary with a Compact Companion in a 44-minute Orbit

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OWJ074106.0-294811.0 with an orbital period of P orb = 44.66279 ± 1.16 × 10 -4 minutes, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, T eff = 39 400 ± 500 K and log g = 5.74 0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass M sdOB = 0.23 ± 0.12 M o with a WD companion (MWD = 0.72 ± 0.17 M o ). The mass ratio was found to be q = M sdOB /MWD = 0.32 ± 0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of ≈ 0.47 M o . To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T eff , log g, and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best-fitting model is a helium white dwarf with a mass of 0.320 M o , which left the common envelope ≈1.1 Myr ago, which is consistent with the observations. As a helium white dwarf with a massive white dwarf companion, the object will reach contact in 17.6 Myr at an orbital period of 5 minutes. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.